Use llvm::endianness::{big,little,native} (NFC)

Note that llvm::support::endianness has been renamed to
llvm::endianness while becoming an enum class. This patch replaces
{big,little,native} with llvm::endianness::{big,little,native}.

This patch completes the migration to llvm::endianness and
llvm::endianness::{big,little,native}.  I'll post a separate patch to
remove the migration helpers in llvm/Support/Endian.h:

  using endianness = llvm::endianness;
  constexpr llvm::endianness big = llvm::endianness::big;
  constexpr llvm::endianness little = llvm::endianness::little;
  constexpr llvm::endianness native = llvm::endianness::native;